Studies of an optical imaging apparatus for causing light irradiated to a specimen from a light source, such as a laser, to propagate inside the specimen and for obtaining information regarding the interior of the specimen have been positively progressed primarily in the medial field. As one of optical imaging techniques based on those studies, Photoacoustic Tomography (PAT) has been proposed. See, for example, U.S. Pat. No. 5,840,023 entitled “Optoacoustic Imaging for Medical Diagnosis” (hereafter referred to as “Patent Literature 1”).
In the Photoacoustic Tomography technique, pulsed light is first generated from a light source and irradiated to a specimen. The irradiated light is caused to propagate and diffuse inside the specimen, while body tissues absorb energy of the light and generate acoustic waves (typically ultrasonic waves) from the same. The generated acoustic waves are detected at plural points located in surrounding relation to the specimen, and obtained signals are mathematically analyzed and processed into image data. Such processing is referred to as imaging hereinafter. An initial pressure generation distribution or an optical characteristic value distribution, particularly, a light-energy absorption density distribution, inside the specimen can be obtained with the imaging, and information regarding the interior of the specimen, such as the position of a malignant tumor, can be obtained.